Parser generator errors when building yosys on macOS

frontends/ilang/ilang_parser.y:46.14-38: syntax error, unexpected string, expecting =

If you get this error when building yosys, your version of Bison is too old. This happened to me on a macOS 10.13.3 system when I was setting up Icestorm. If you’re using a Mac:

brew install bison

Homebrew doesn’t symlink the new version, so pass it explicitly to Make.

BISON=/usr/local/opt/bison/bin/bison make -j4